Title:
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Finite Element Analysis and design of Ferrite Phase Shifters
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Signal routing at Watts and KiloWatts is primarily handled by using
waveguides or stripline structures containing magnetized ferrites. Over the last.
50 years, many ferrite phase shift and control devices have been developed
across the microwave spectrum for a wide range of power levels and now
commercially available. However, many challenges remain to be addressed if
this body of knowledge is to be extended to meet demands for miniaturization,
broader relative bandwidths, and higher operating frequencies at reduced costs.
Ferrite technology still remains to a great extent "a black art" because the
design of ferrite phase shift and control devices is primarily based on the
empirical approximations, circuit models and extensive experimentation. In this
work a non-Hermitian gyro tropic finite element formulation is presented for
the modelling of gyromagnetic structures. The tensor permeability expressions
which are incumbent for the proper characterization of gyromagnetic
components have been derived and used in the derivation of a generic finite
element (FE) functional for gyromagnetic components. The functional is
developed from Maxwell's coupled equations before demonstrating its
stationary properties at the boundary value problem solution. The formulation
is implemented with finite element method to ensure that only physical eigen
solutions exist. This four-transverse field functional formulation evaluates
complex propagation constant as its eigen value.
The method is then applied to design a twin toroidal ferrite phase shifter with a
low reluctance magnetic bias circuit. Viscous plastic processing method (VPP) is
used to fabricate ferrite toroids to ensure a homogeneous grain structure with
continuous shape. VPP fabrication technique confers improved ferrite
properties with reduce low-field loss and threshold for spinwave instability;
this is imperative in high quality, partially magnetized devices. A closed form
field formulation is used to design matching quarter wavelength impedance
transformers by calculating the fields and dispersion expressions for the
matching section. This compact and power efficient devi~e was then tested. The
insertion loss was less than IdB over the frequency band from 9.5-10.3GHz and
return loss of 20dB was achievable. The phase shift calculations agree to within
10% of the measured values.
The variational finite element formulation developed in chapter 5, is further
used to design and fabricate a high power partial height ferrite phase shifter. At
high power levels nonlinear loss is prevalent but this has been avoided using a
technique called "mode segregation" by biasing the ferrite above ferrimagnetic
resonance. The above. resonance operation requires larger external bias fields.
The calculations agree well with the experimental data.
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